Angle-resolved Raman spectroscopy (ARRS) is an effective method to analyze the symmetry of phonons and other excitations in molecules and solid-state crystals. While there are several configurations of ARRS instruments, the measurement system detailed here utilizes two pairs of linear polarizers and superachromatic half-wave plates. After the orientations of the linear polarizers are set to fixed angles, the two half-wave plates rotate independently, through motorized control, enabling 2D linear polarization mapping. Described within is a protocol to achieve high quality ARRS measurements leveraging phonons from easily accessible test materials [molybdenum disulfide (MoS_2), sapphire (Al_2O_3) and silicon] to validate the system and operation. Quantitative polarized Raman data strongly depends on the quality of sample surface and the optics: the order of placement, alignment, and any distortion caused by their coatings. This study identifies the impact of commonly used edge filters on the polarization response of materials with an anisotropic response as emulated by the T_2g phonon in the Si(100). We detect and model the significant distortion of the T_2g phonon polarization response originating from our dichroic edge filters, the results of which are broadly applicable to optics in any Raman instrument. This ARRS setup also enables helicity-resolved Raman measurements by replacing the first half-wave plate with a superachromatic quarter-wave plate; this configuration is also validated using the Raman response of the aforementioned test materials. This paper aims to increase the quality and reproducibility of polarized Raman measurements through both instrumental considerations and methodology.

The capability to specifically assemble aligned anisotropic nanotube structures is highly desirable for applications, whether for wide bandgap boron nitride nanotubes (BNNTs) or single-wall carbon nanotube (SWCNTs). In this effort [1], we demonstrate the first use of slow vacuum filtration (SVF) for the production of aligned BNNT thin films, producing well-aligned films of high BNNT purity and visible wavelength transparency. SVF is then further utilized for the production of co-composite aligned films of semiconducting SWCNTs and BNNTs at different mass ratios, demonstrating templated co-alignment of the SWCNTs at small mass fraction by the majority BNNT fraction, and enabling identification of reasonably optimized process parameters. The templated inclusion of semiconducting SWCNTs is particularly interesting because of the different bandgaps of the two materials, enabling additional visible and near-infrared polarized optical spectroscopy characterizations of alignment, and for the effective dilution of individual aligned SWCNTs in an inert matrix. Characterization of alignment is demonstrated via birefringence, spectroscopic linear dichroism, and polarized Raman scattering. Dilution of aligned pure (6,5) SWCNTs in a BNNT matrix is further demonstrated to dramatically increase fluorescence yield relative to SWCNT only films, demonstrating a possible method for improving optical performance in optical coupling and for decoupling positioning and alignment of individual SWCNTs in devices such as demonstration single photon sources. In this talk I will describe the production, optimization, and characterization of co-composite BNNT-SWCNT films using SVF and the importance of controlling nanotube characteristics for well-controlled production. [1]: Shapturenka, P.; Adel, T.; Abel, F. M.; Hight Walker, A. R.; Fagan, J. A.Aligned Boron Nitride Nanotube Thin Films and their Co-Composites with Single Wall Carbon Nanotubes through Slow Vacuum Filtration. ACS Nanoscience AU 2025 , 5(4), 293–305.doi.org/10.1021/acsnanoscienceau.5c00022

Boron nitride nanotubes (BNNTs) are a promising nanomaterial due to their remarkable optical and mechanical properties, chemical robustness, and extended aspect ratios. Herein, we report the formation of strongly biaxially aligned thin films of BNNTs using automated slow vacuum filtration (SVF), as well as their cocomposites with single-wall carbon nanotubes (SWCNTs). Pure BNNT SVF-generated films are found to differ in optimization conditions from those identified previously for SWCNTs but display similar improvements in alignment and uniformity with advanced purification for nanotube length and homogeneity, with globally aligned films observed. Mixed, cocomposite, biaxially aligned films of BNNTs with SWCNTs are also described. Such films provide effective and efficient hosting capabilities for unique morphologies of distributed and individualized SWCNTs aligned by a wide-bandgap BNNT matrix. Concentrations upward of 25% SWCNT mass fraction were found to reside within majority-BNNT films without significantly disrupting the global composite structure; the SWCNT fraction, in turn, enabled probing of both local and global nematic alignment through their use as spectroscopic reporters. Leveraging the thickness and alignment control provided by our SVF implementation, both neat BNNT and composite films show great promise for advancing novel photonic and other thin-film nanocomposite applications requiring tailorable mechanical, thermal, optical, and electronic functionalities.

Two-dimensional (2D) 1T-VSe2 has prompted significant interest due to the discrepancies regarding alleged ferromagnetism (FM) at room temperature, charge density wave (CDW) states, and the interplay between the two. We employed a combined Diffusion Monte Carlo (DMC) and density functional theory (DFT) approach to accurately investigate the magnetic properties, CDW states, and their responses to strain in monolayer 1T-VSe2. Our calculations show the delicate competition between various phases, revealing critical insights into the relationship between their energetic and structural properties. We performed classical Monte Carlo simulations informed by our DMC and DFT results and found the magnetic transition temperature (Tc) of the undistorted (non-CDW) FM phase to be 228 K and the distorted (CDW) phase to be 68 K. Additionally, we studied the response of biaxial strain on the energetic stability and magnetic properties of various phases of 2D 1T-VSe2 and found that small amounts of strain can increase the Tc, suggesting a promising route for engineering and enhancing magnetic behavior. Finally, we synthesized 1T-VSe2 and performed Raman spectroscopy measurements, which were in close agreement with our calculated results, validating our computational approach. Our work emphasizes the role of highly accurate DMC methods in advancing the understanding of monolayer 1T-VSe2 and provides a robust framework for future studies of 2D magnetic materials.

Lateral resolution is a key figure of merit for spectroscopy across all applications. Confocal Raman spectroscopy is able to provide chemical and structural information with submicrometre resolution, resulting in widespread use across multiple disciplines of science and technology. However, the lack of agreed-upon measurement standards and appropriate reference samples has hindered uptake. Here, we report the development and demonstration of a reference sample based on indium arsenide (InAs) semiconducting nanowires for measuring the lateral resolution of confocal Raman spectroscopy with a pathway for traceability to the International System of Units (SI). An interlaboratory comparison involving 15 participants from 11 countries has been conducted to rigorously test and demonstrate the suitability of the sample and the method. The study identified required revisions to the measurement protocol to improve the consistency of data analysis and that the long-term operational stability of the reference sample requires further improvement. Based on a revised data analysis protocol, the method delivered consistent results at the 95% confidence level for eight of the nine participants who returned full datasets. Outcomes from this study have contributed to the publication of a new international standard (ISO 23124:2024).

RuO_2 has been proposed as the prototypical altermagnetic material. However, several reports have recently questioned its intrinsic magnetic ordering, leading to conflicting findings, especially in thin film heterostructures pointing to possible interface effects being convoluted with supposed antiferromagnetic/altermagnetic signatures. Here, extensive magnetometry measurements were performed on two independently grown thin film heterostructures of RuO_2 interfaced with either NiFe or Fe acting as the ferromagnetic layer. Below about 15 K, both samples exhibit exchange bias fields when cooled to approximately 2 K in a +1 T field, and a spin transitional feature is observed around 31 K. Magneto-Raman measurements on RuO_2 thin films only reveal a magnon mode when there is a NiFe layer, suggesting that RuO_2 does not intrinsically possess long range magnetic ordering.. When in contact with a ferromagnet, RuO_2 displays effects that could be ascribed to antiferromagnetism. However, the lack of intrinsic magnon modes points toward possible diffusion between the layers or spin disorder at the interface as seen by density functional theory (DFT) calculations.

The entanglement of electronic spin and orbital degrees of freedom is often the precursor to emergent behaviors in condensed matter systems. With considerable spin-orbit coupling strength, the cobalt atom on a honeycomb lattice offers a platform that can make accessible the study of novel magnetic ground states. Using temperature-dependent Raman spectroscopy and high-magnetic field Raman and infrared (IR) spectroscopy, we studied the lattice and spin-orbital excitations in CoTiO_3, an antiferromagnetic material that exhibits topologically protected magnon Dirac crossings in the Brillouin zone. Under the application of an external magnetic field up to 22 T along the crystal's c-axis, we observed the splitting of both the spin-orbital excitations and a phonon nearby in energy. Using density functional theory (DFT), we identify a number of new modes that below the antiferromagnetic (AFM) transition become Raman-active due to the zone-folding of the Brillouin zone caused by the doubling of the magnetic unit cell. We use a model that includes both the spin and orbital degrees of freedom of the Co^2+ ions to explain the spin-orbital excitation energies and their behavior in an applied field. Our experimental observations along with several deviations from the model behavior point to significant coupling between the spin-orbital and the lattice excitations.

Two-dimensional (2D) 1T-VSe$_2$ has prompted significant interest due to the discrepancies regarding alleged ferromagnetism (FM) at room temperature, charge density wave (CDW) states and the interplay between the two. We employed a combined Diffusion Monte Carlo (DMC) and density functional theory (DFT) approach to accurately investigate the magnetic properties and response of strain of monolayer 1T-VSe$_2$. Our calculations show the delicate competition between various phases, revealing critical insights into the relationship between their energetic and structural properties. We went on to perform Classical Monte Carlo simulations informed by our DMC and DFT results, and found the magnetic transition temperature ($T_c$) of the undistorted (non-CDW) FM phase to be 228 K and the distorted (CDW) phase to be 68 K. Additionally, we studied the response of biaxial strain on the energetic stability and magnetic properties of various phases of 2D 1T-VSe$_2$ and found that small amounts of strain can enhance the $T_c$, suggesting a promising route for engineering and enhancing magnetic behavior. Finally, we synthesized 1T-VSe$_2$ and performed Raman spectroscopy measurements, which were in close agreement with our calculated results. Our work emphasizes the role of highly accurate DMC methods in advancing the understanding of monolayer 1T-VSe$_2$ and provides a robust framework for future studies of 2D magnetic materials.

The magnetic interactions that determine magnetic order and magnon energies typically involve only two spins. While rare, multi-spin interactions can also appear in quantum magnets and be the driving force in the ground state selection and in the nature of its excitations. By performing time-domain terahertz and magneto-Raman spectroscopy measurements combined with theoretical modeling, we determine the origin of the magnon excitation gap in Dirac antiferromagnet CoTiO$_3$. By adding a ring-exchange interaction in a hexagonal plaquette of the honeycomb lattice to both an XXZ spin model and to a low energy spin-orbital flavor wave model, a gap is generated in the magnon spectrum at the Brillouin zone center. With this addition, the flavor wave model reproduces a large swath of experimental results including terahertz, Raman, inelastic neutron scattering, and magnetization experiments.

Chemical vapour deposition (CVD) synthesis of graphene on copper has been broadly adopted since the first demonstration of this process1. However, widespread use of CVD-grown graphene for basic science and applications has been hindered by challenges with reproducibility2 and quality3. Here we identify trace oxygen as a key factor determining the growth trajectory and quality for graphene grown by low-pressure CVD. Oxygen-free chemical vapour deposition (OF-CVD) synthesis is fast and highly reproducible, with kinetics that can be described by a compact model, whereas adding trace oxygen leads to suppressed nucleation and slower/incomplete growth. Oxygen affects graphene quality as assessed by surface contamination, emergence of the Raman D peak and decrease in electrical conductivity. Epitaxial graphene grown in oxygen-free conditions is contamination-free and shows no detectable D peak. After dry transfer and boron nitride encapsulation, it shows room-temperature electrical-transport behaviour close to that of exfoliated graphene. A graphite-gated device shows well-developed integer and fractional quantum Hall effects. By highlighting the importance of eliminating trace oxygen, this work provides guidance for future CVD system design and operation. The increased reproducibility and quality afforded by OF-CVD synthesis will broadly influence basic research and applications of graphene. Assessment of surface contamination shows that trace oxygen is a key factor influencing the trajectory and quality of graphene grown by low-pressure chemical vapour deposition, with oxygen-free synthesis showing increased reproducibility and quality.

This article presents an interlaboratory comparison (ILC) on Raman spectroscopy as a technique for relative quantification of the two most common polymorphs of titanium dioxide (TiO2)—anatase and rutile—in binary mixtures. Some standard methods are currently employed internationally for the determination of TiO2 content in samples (ISO 591-1, ASTM D3720-90), but require extensive sample preparation, do not distinguish between the two polymorphs or are accurate only for small fractions of either polymorph. Raman spectroscopy is a well-suited characterization technique for measuring and differentiating TiO2 in a fast, non-invasive way, while requiring no particular reagent or sample preparation. Eleven international participants conducted the study under the framework of Versailles Project on Advanced Materials and Standards. The collected data was analyzed by means of partial least squares regression after spectral preprocessing. The resulting models all show discrepancies of lower than 2% from the nominal values in the quantitative analysis over the concentration range of 5%–95% mixture fractions, with many datasets showing substantial improvement margins on this figure. The results of this ILC provide validation of Raman spectroscopy as a reliable method for quantification of TiO2 phases.

Powders and films composed of tin dioxide (SnO2) are promising candidates for a variety of high-impact applications, and despite the material's prevalence in such studies, it remains of high importance that commercially available materials meet the quality demands of the industries that these materials would most benefit. Imaging techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), were used in conjunction with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) to assess the quality of a variety of samples, such as powder and thin film on quartz with thicknesses of 41 nm, 78 nm, 97 nm, 373 nm, and 908 nm. In this study, the dependencies of the corresponding Raman, XPS, and SEM analysis results on properties of the samples, like the thickness and form (powder versus film) are determined. The outcomes achieved can be regarded as a guide for performing quality checks of such products, and as reference to evaluate commercially available samples.

Dead-end filtration on polyvinylpyrrolidone-coated polycarbonate track-etched membranes has proven to be an effective method to prepare macroscopically (3.8 cm²) aligned thin films from solution-based single-wall carbon nanotubes (SWCNTs). However, to make this technique broadly applicable, the role of SWCNT length and diameter on the alignment process must be understood. To date, most groups report the alignment of unsorted large diameter (» 1.4 nm) SWCNTs, but systematic studies on their small diameter (» 0.78 nm) counterparts are rare and in the case of sorted SWCNT species, non-existent. In this work, films are prepared from two series of length sorted fractions comprised of small and large diameter SWCNTs, respectively. The degree of alignment is characterized by cross polarized microscopy, scanning electron microscopy and Raman spectroscopy. For the longest fractions, the two-dimensional order parameter, , values of » 0.66 and » 0.76 are reported for the small and large diameter SWCNTs, respectively, and Raman maps over an area of 2500 μm 2 are presented. A comparison of Derjaguin, Landau, Verwey and Overbeek (DLVO) theory calculations with the aligned domain size is then used to propose a law identifying the required length of a carbon nanotube with a given diameter and zeta potential for alignment.

Covalent modification of carbon nanotubes is a promising strategy for engineering their electronic structures. However, keeping modification sites in registration with a nanotube lattice is challenging. We report a solution using DNA-directed, guanine (G)-specific cross-linking chemistry. Through DNA screening we identify a sequence, C 3 GC 7 GC 3 , whose reaction with an (8,3) enantiomer yields minimum disorder-induced Raman mode intensities and photoluminescence Stokes shift, suggesting ordered defect array formation. Single-particle cryo–electron microscopy shows that the C 3 GC 7 GC 3 functionalized (8,3) has an ordered helical structure with a 6.5 angstroms periodicity. Reaction mechanism analysis suggests that the helical periodicity arises from an array of G-modified carbon-carbon bonds separated by a fixed distance along an armchair helical line. Our findings may be used to remodel nanotube lattices for novel electronic properties.

Renewable energy sources are an attractive solution for reducing greenhouse gas emissions; yet, these renewable energies require energy storage. Among large-scale energy storage systems, the iron redox flow battery is a candidate because of its low cost, long lifetime, and safety. Aqueous interfaces are the gateway to many electrochemical reactions, including those that involve soluble Fe(III). However, interfacial iron complexation and iron surface prevalence are not well understood, especially under inherently acidic conditions of FeCl3 solutions. In this study, we investigate interfacial Fe(III) species using second harmonic generation (SHG) combined with surface tension and UV/visible absorption. Surface-selective techniques such as SHG have been widely applied to unveil the unique surface properties within the air-aqueous interface. Here, we perform SHG with excitation at 760, 800, and 860 nm wavelengths. For 800 and 860 nm, the kinetics of interfacial FeCl3 molecules follows the Frumkin-Fowler-Guggenheim adsorption model. For 760 nm, we observe two different interfacial concentration regimes marked by two distinctly different SHG trends. Below a 3.0 mol/kg water concentration of FeCl3, nonresonant behavior is observed, which is similar to that from an aqueous sodium iodide surface but much larger in magnitude than aqueous NaCl and NaBr solution surfaces. Above 3.0 mol/kg water, a dramatic increase in the SHG slope is observed. Through evaluation of centrosymmetry and SHG resonance, we propose the existence of the neutral [FeCl3(H2O)3] complex at the air-aqueous interface.

We report on the temperature evolution of the polarization-dependent Raman spectrum of exfoliated MoI$_3$, a van der Waals material with a "true one-dimensional" crystal structure that can be exfoliated to individual atomic chains. The temperature evolution of several Raman features reveals anomalous behavior suggesting a phase transition of a magnetic origin. Theoretical considerations indicate that MoI$_3$ is an easy-plane antiferromagnet with alternating spins along the dimerized chains and with inter-chain helical spin ordering. The calculated frequencies of the phonons and magnons are consistent with the interpretation of the experimental Raman data. The obtained results shed light on the specifics of the phononic and magnonic states in MoI$_3$ and provide a strong motivation for future study of this unique material with potential for spintronic device applications.

We report the polarization-dependent Raman spectra of exfoliated MoI3, a van der Waals material with a “true one-dimensional” crystal structure that can be exfoliated to individual atomic chains. The temperature evolution of several Raman features reveals an anomalous behavior suggesting a phase transition of magnetic origin. Theoretical considerations indicate that MoI3 is an easy-plane antiferromagnet with alternating spins along the dimerized chains and with inter-chain helical spin ordering. The calculated frequencies of phonons and magnons are consistent with the interpretation of the experimental Raman data. The obtained results shed light on the specifics of the phononic and magnonic states in MoI3 and provide a strong motivation for further study of this unique material with potential for future spintronic applications.

Surface potential measurement values of the gas-liquid interface can be ambiguous despite the numerous electrochemical approaches used for quantification of the reported values. Calibration and normalization methods can be undefined, which often undermines the robustness of the reported values. Surface potential instrumentation and data interpretation also varies significantly across literature. Here, we propose a circuit model for an ionizing surface potential method based on the alpha decay of a radioactive americium-241 electrode. We evaluate the robustness of the circuit model for quantifying the surface potential at the air-aqueous interface. We then show successful validation of our circuit model through determination of the surface tension of the air-electrolyte interface with comparison to respective surface tension literature values. This validation reveals the reliability of surface potential measurements using the americium-241 ionizing method.

Complementing the microscopic picture of the surface structure of electrolyte solutions set out by previous theoretical and experimental studies, the ionizing surface potential technique offers a unique approach to quantifying the impact of aqueous inorganic ions upon the interfacial electric field of the air-aqueous interface. In this Feature Article, we review the vulnerability of theoretical and empirically derived χwater values as a normative reference for aqueous ion surface potentials. Instead, we recognize and evaluate aqueous ion surface potentials relative to well-known ionic surfactants cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Additionally, we also explore factors that impact the magnitude of the measured surface potentials using the ionizing method, particularly in the type of reference electrode and ionizing gas environment. With potential measurements of sodium halide solutions, we show that iodide has a dominant effect on the air-aqueous electric field. Compared to chloride and bromide, iodide is directly observed with a net negatively charged surface electric field at all salt concentrations measured (0.2 to 3.0 mol/kg water). Also, above the 2 M region, bromide is observed with a net negatively charged surface. Although several scenarios contribute to this effect, it is most likely due to the surface enrichment of bromide and iodide. While the results of this study are pertinent to determining the specific interfacial reactivity of aqueous halides, these anions seldom transpire as single-halide systems in the natural environment. Therefore, we also provide an outlook on future research concerning surface potential methods and more complex aqueous electrolyte systems.

Aqueous solutions of FeCl3 have been widely studied to shed light on a number of processes from dissolution, mineralization, biology, electrocatalysis, corrosion, to microbial biomineralization. Yet there are little to no molecular level studies of the air-liquid FeCl3 interface. Here, both aqueous and glycerol FeCl3 solution surfaces are investigated with polarized vibrational sum frequency generation (SFG) spectroscopy. We also present the first ever extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy measurements of solvated ions and complexes at a solution interface, and observe with both X-ray photoelectron spectroscopy (XPS) and XUV-RA the existence of Fe(III) at the surface and in the near surface regions of glycerol FeCl3 solutions, where glycerol is used as a high vacuum compatible proxy for water. XPS showed Cl- and Fe(III) species with significant Fe(III) interfacial enrichment. In aqueous solutions, an electrical double layer (EDL) of Cl- and Fe(III) species at 0.5 m FeCl3 concentration is observed as evidenced from an enhancement of molecular ordering of water dipoles, consistent with the observed behavior at the glycerol surface. At higher concentrations in water, the EDL appears to be substantially repressed, indicative of further Fe(III) complex enrichment and dominance of a centrosymmetric Fe(III) species that is surface active. In addition, a significant vibrational red-shift of the dangling OH from the water molecules that straddle the air-water interface reveals that the second solvation shell of the surface active Fe(III) complex permeates the topmost layer of the aqueous interface.